The present invention relates to a hydrogen separator. More particularly, the present invention relates to a hydrogen separator which is superior in durability and has good hydrogen permeability, etc.
A hydrogen separator comprising a porous ceramic substrate and a hydrogen-separating layer made of palladium or the like, formed on the substrate has been used in order to separate only hydrogen selectively from a hydrogen-containing gas such as steam-reformed gas or the like. Such a hydrogen separator is used, in some cases, for selection of only hydrogen at high temperatures. Therefore, the hydrogen separator is required to have high gas tightness at high temperatures or in an environment where temperature increase and decrease is repeated.
As shown in FIG. 2, ordinary hydrogen separators have a structure comprising a porous substrate 12 and a hydrogen-separating layer 13 disposed on the first surface 5 of the porous substrate 12 in a state that the hydrogen-separating layer has a penetrated portion 17 formed so as to extend from the first surface 5 to a particular depth (the penetration depth D2 of the penetrated portion). Incidentally, the porous substrate 12 has a large number of pores communicating from the first surface 5 to the second surface (not shown) and is constituted by, for example, ceramic particles. In producing a hydrogen separator 11 having a structure shown in FIG. 2, disposition of a hydrogen-separating layer 13 can be conducted, for example, by the plating methods with pressure difference disclosed in the following patent literatures 1 to 4.
Patent literature 1: JP-3213430
Patent literature 2: JP-A-2003-190748
Patent literature 3: JP-A-1998-113545
Patent literature 4: JP-2000-317282
Incidentally, in these plating methods with pressure difference, the diameters of the pores at the first surface 5 of the porous substrate 12 are set small and the hydrogen-separating layer 13 is formed as a thin film, whereby generation of defects such as cracks and the like in the hydrogen-separating layer 13 is prevented.
However, when the pore diameters of the porous substrate are too small and if the penetration depth of the penetrated portion formed is too large, there is a problem that permeation of hydrogen present in a to-be-treated gas mixture is prevented and no good hydrogen permeability is obtained. Meanwhile, if the penetration depth of the penetrated portion formed is too small, there is a problem that the adhesivity between the hydrogen-separating layer and the porous substrate is low, the hydrogen-separating layer tends to peel off, and the hydrogen separator is low in durability.